Amplifier gain control



NOV. 18, 1941.

' w. VAN B. ROBERTS AMPLIFIER GAIN CONTROL Filed Jan.'

\WJ. J a

INV EN TOR. ALTER KM 13. ROBERTS I ATTORNEY.-

Patented Nov. 18, 1941 AMPLIFIER GAIN CONTROL Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 31, 1940, Serial No. 316,476

7 Claims.

My present invention relates to gain control circuits for amplifiers, and more particularly to an automatic amplifier gain control circuit of minimum signal distortion.

It is known in the prior art to provide a signal amplifier tube with an auxiliary anode which functions as a so-called waste electrode, and to control the amplifier gain by diverting a variable fraction of the electron stream from the output electrode, to the Waste electrode while keeping the total cathode current constant. In this mode of gain control the operating characteristic-of the working section of the tube is altered solely in the scale of output current, and notin the shape of the curve. In this way'it is possible variably to amplify weak signals while operating at all times on the most linear portion of the tube characteristic curve.

According to my present invention there is provided a simple tube structure for securing the aforesaid distortionless gain control, and the entire circuit being well'adapted to different types of gain control'action.

It may be stated that it is an important object of my invention to provide a signal amplifier provided with a control grid assembly capable of producing a plurality of sheets, or beams, of electrons, and an anode structure similar in structure, and so related, to the grid assembly that upon appropriate biasing of the grid assembly the beams may be variably deflected between a pairof independent systems of the anode assembly.

Still other objects ofthe invention are to improvegenerally the efliciency and simplicity of automatic gain control circuits, and more especially to provide such circuits in an economical fashion. j

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, there is shown a signal detection system providing an audio voltage amplifier embodying the,

invention. While the invention is not limited in any way to audio frequencies, the specific illustration is of an audio amplifier since it is considered that the ideal gain control should operate on the audio amplifiers without distortion.

Hence, numeral l denotes a rectifier, such as one of the diode type, having its anode 2 connected to the high potential side of resonant input circuit 3. The cathode 4 is connected to the low potential side of input circuit 3 through a load resistor 5 shunted by a carrier frequency bypass condenser 6. The midpoint of resistor 5 is grounded so that the anode and cathode ends of the resistor may assume increasingly negative and positive potentials with carrier amplitude increase.

The input circuit 3 can be coupled to any sig-. nal sources. For example, if the rectifier is embodied in a superheterodyne receiver, the circuit 3 will be tuned to the operating intermediate frequency (I. F.) of the system. The circuit feeding the circuit 3 may be the I. F. output circuit of an I. F. amplifier. However, the system could be a tuned radio frequency receiver, and in such case the circuit 3 would be variably tuned over a signal frequency range. Assuming, then, that audio modulated carrier voltage is rectified, there will appear across the load resistor 5 modulation voltage and direct current voltage. I The modulation voltage is impressed upon an audio amplifier network including at least one tube 1 constructed as shown in the drawing.

The tube electrodes have been greatly magnified in order to show clearly the manner of construction of the electrodes. Within the tube envelope is an electron emission element 8, such as the usual tube cathode, and the latter is surrounded by a control grid assembly composed of double, interleaved turns. In other words, the

grid assembly is constructed as a double grid spiral. The grid turns9, shown as small circles, are to be understood as providing one spirallywound grid, whi1e the turns [0 shown as black dots of equal diameter provide a second interleaved spirally wound grid.

Cathode 8 is connected to ground. through a usual biasing resistor ll shunted by a bypass condenser [2. Grid I0 is connected by lead l3 and filter resistor 14 to the negative, or anode, end of resistor 5. The grid 9 is connected by lead l5 and filter resistor Hi to the cathode, or positive, end of resistor 5. These connections i 3l5 provide the direct current voltage, or automatic biasing, connections to grids 9 and Ill. The normal bias of grids 9 and I0 is established by bias voltage developed across resistor II. In

the absence of carrier voltage grids 9 and to are at the same negative potential equal to the normal bias, and, therefore, exert equal control. on

the electron beams, or sheets of electrons, passing between successive grid turns.

Audio modulation voltage developed across resistor is impressed equally and cophasally upon grids 9 and ID by a slidable tap l6 connected between the resistor 5 and the junction of condensers I1 and IS. The latter are in series between the grid ends of resistors l4 and 16. The condensers l1 and [8 have, a low impedance to audio voltage components, but block the direct current voltage components. The slidable tap l6 may be employed between ground and either: half of resistor 5. It will be observed that grids 9 and H) are arranged in a manner similartoknown coplanar grids. a 7 1 The anode structure, or assembly,'consists of a pair of similarly arranged interleaved turns. For example, numeral '20 denotes the turns of the waste electrode, while numeral'2l designates the turns of the useful output electrode, I The turns and 2|, therefore, provide apair of interleaved, spirally wound turns concentric I 1 with-the interleaved, grids 9 and l0. The arrangement is 'such thatfeach'of turns 29 is 'in substantial alignment with each'turn lfl and 9, while each turn 2] is in substantial alignment with a linefpassing half way between successive turns 9 and I0. Numeral 39 designates arrows which should be understood as representing normal paths of the electrons in each sheet, or beam,

of electrons. It will be noted that the electrons are directed at the turnsof' output electrode 2! when the grids 9 and IU' havethe normal equal negativ'ebiases appliedthereto.

The output electrode 2l is connected Ito a,

source of positive potentialv through a load r'esistor 40, and the amplified voltage is transmit- I ted'through audio coupling condenser 4| to suc; ceeding audio voltage utilization circuits, The

waste electrode 29 is connected to a positive voltage source by a resistor 50. It is to be understood that the positive potentials applied to electrodes 20 and 2| will be so relatively proportioned that, with normal biases on grids 9 and In, the beams 30 will be formed and concentrate, or impinge, on the output electrode 2|. This would be the maximum, amplification, or weak signal, state of amplifier 1. Assuming, now, that the carrier amplitude at the carrier input'circuit increased in magnitude,

r affected.

fact that the total cathode current is maintained substantially constant. It will, therefore, be seen that the electron beam deflection means is a double spiral control device, the turns of one group being insulated from the turns of the other. Each group of turns, or each control electrode, has a separate biasing connection, although acommon bias voltage could be applied to both control grids. 'The impedances' 40 and 59 may be made similar -in order to preserve the desired distribution of current between the two anode groups; So long as the signal voltage is. small compared to the bias applied to the control electrodes the beaming action'is not much Of course, negative voltage at the anode end of resistor 5 'may be used as volume control bias,.for amplifier stages preceding detector l. Similarly, the positive voltage at the cathode end' ofyresistor 5 could be used for gain expansion control of any amplifier of the reeliminated and the lbiasconnection l5 onlyused gain reduction, would alsobe had. Qf co urse, "in either case, the entire drop" across resistor, 5

couldbeused. I I Automatic gain expansion is had by simply.

I reversing the connections of output loads and the, grid 9 would be biased in a positive polarity.

sense, while grid I'll would be made increasing- 1y negative. The turns ofgrids 9 and lflwould act as beam deflection elements, since each turn of grid It] would repel its adjacent beam from the impinged turn of output electrode 2| towards an adjacent turn of electrode 20. In the same Way each turn of grid 9 would attract, or'draw, its adjacent beams towards the turnsof electrade 20 in alignment therewith. I'he dotted arrows 30' show the manner in which a pair of beams are deflected towards, or diverted to, a turn of electrode 20 by the attraction exerted by the positively biased electrode 9, and the repullsion exercised by the negatively biased electrode l0.

The diversion of the electron beams to the turns of the waste electrode 20 increases with increasing carrier amplitudes. Hence, the audio gain would be .correspondingly decreased. The

rate of diversion, as Well as the extent thereof, I

can be controlled to vary the output volume level in any predetermined manner. The advantage of varying the biases of both control electrodes 9. and I 0 in differential manner resides in? the 40 to output turns 29 and. 21. By connecting resistor 50 to electrodeturns il, and the-output resistor 49 toturnslfl, at Si nal, voltage the output would be small, and would increasewith signal voltage .input increase". ,ByappIying audio,

signal voltage, in this latter; expansion function,

to grids 9 and I0, and-rectifying the audio volty age to secure a direct current voltagedependpansion is secured.

ent on modulation percentage, audio gaingex Signal potentials developedon the two groups of anodes will always tend to, oppose the de-, flection of the beams. I This action may be, welcomed as a form of'degenerativefeedback ori: load reaction, or it may be reduced by' interpose,

ing an, ordinary screen grid between the-grids 9i0 and theanodes 292|. In that-case the turns 6!] of the screen grid are locatedmidway between eachpair of successive anode turns, and, slightly nearer the cathode so that theybeamsf are shielded from the influenceof one set of anode turns after they have passed the) screen wires on the side nearestthe other set of anode turns. a I While I have indicated'and described a system for carrying my inventionintoeffectlit will be apparent to one skilled in, the artthat'my in,- vention is by no means 'limitedto the particular organization shown and described, but that many modifications may ,bemade without depart,-. ing from the scope of myinvention, as set, forth in the appended claims.

What I claim is:

1. In a signal amplifier network, a tube provided with an electron emission element, a plurality of double interleaved anodeelements, an

output load impedance connectedto, one set or said anode elements, and'a plurality of double;

interleaved electron deflection electrode elements arranged in a spaced relation similar to one set of said anode elements, means automatically responsive to signal intensity variations for varying the direct current potential of at least one set of said deflection electrode elements, and other means for applying signal voltage to at least the remainder of said deflection electrode elements.

2. In a signal amplier circuit, an electron discharge tube provided with a cathode, a pair of anode electrodes, and a pair of electron deflection electrodes, said deflection electrodes each consisting of a plurality of spaced control elements, and the control elements of one deflection electrode being interleaved with the control elements of the other deflection electrode, said anode electrodes each consisting of a plurality of anode elements, and the anode elements of each anode being interleaved with the anode elements of the other anode, the anode elements of one anode electrode being in substantial alignment with the control elements of said deflection electrodes whereby a plurality of electron beams are provided, said beams flowing to the anode elements of the second one of said anodes, means for applying signal voltages to said deflection electrodes, and means adjusting the direct current potential of said deflection electrodes for deflecting said electron beams from the anode elements of said one anode to the anode elements of the second anode.

3. In a signal amplifier circuit, an electron discharge tube provided with a cathode, a pair of anode electrodes, and a pair of electron deflection electrodes, said deflection electrodes each consisting of a plurality of spaced control elements, and the control elements of one deflection electrode being interleaved with the control elements of the other deflection electrodei, said anode electrodes each consisting of a plurality of anode elements, and the anode elements of each anode being interleaved with the anode elements of the other anode, the anode elements of one anode electrode being in substantial alignment with the control elements of said deflection electrodes whereby a plurality of electron beams are provided, said beams flowing to the anode elements of the second one of said anodes, means for applying signal voltages to said deflection electrodes, means for deflecting said electron beams from the anode elements of said one anode to the anode elements of the second anode, and said last means comprising a source of signal-derived direct current voltages which is connected to both deflection electrodes to differentially bias them.

4. In a signal amplifier circuit, an electron discharge tube provided with a cathode, a pair of anode electrodes, and a pair of electron deflection electrodes, said deflection electrodes each consisting of a plurality of spaced control grid elements, and the control elements of one deflection electrode being interleaved with the control elements of the other deflection electrode, said anode electrodes each consisting of a plurality of anode grid elements, and the anode elements of each anode being interleaved with the anode elements of the other anode, the anode elements of one anode electrode being in substantial alignment with the control elements of said deflection electrodes whereby a plurality of electron beams are provided, said beams flowing to the anode elements of the second one of said anodes, means for applying signal voltages to said deflection electrodes, and means adjusting the direct current potential of at least one deflection electrode for deflecting said electron beams from the anode elements of said one anode to the anode elements of the second anode.

5. In a signal amplifier circuit, an electron discharge tube provided with a cathode, a pair of anode electrodes, and a pair of electron deflection electrodes, said deflection electrodes each consisting of a plurality of spaced control elements, and the control elements of one deflection electrode being interleaved with the control elements of the other deflection electrode, said anode electrodes each consisting of a, plurality of anode elements, and the anode elements of each anode being interleaved with the anode elements of the other anode, the anode elements of one anode electrode being in substantial alignment with the control elements of said deflection electrodes whereby a plurality of electron beams are provided, said beams flowing to the anode elements of the second one of said anodes, means for applying signal voltages co-phasally to said deflection electrodes, .and variable direct current voltage means connected to at least one deflection electrode for deflecting said electron beams from the anode elements of said one anode to the anode elements of the other anode.

6. In an alternating voltage transmission system, an electron discharge tube of the type provided with an electron emission element, an output anode electrode, at least two control electrodes positioned adjacent said emission element and arranged to provide an electron beam flowing between said control electrodes to said output electrode, an auxiliary output electrode of at least two elements in alignment with each of said control electrodes, said auxiliary electrode elements being in alignment with said output electrode in a direction substantially normal to said first alignment, load circuit connected to said auxiliary output electrode, means for applying signal currents upon said control electrodes, and means for varying the potential of at least one of the control electrodes to deflect the electron beam from said output electrode to said auxiliary electrode.

7. In a signal transmission system, an electron discharge tube of the type provided with an electron emission element, an output anode electrode, at least two control electrodes positioned adjacent said emission element and arranged to provide an electron beam flowing between said control electrodes to said output electrode, an auxiliary output electrode of at least two elements in alignment with each of said control electrodes, said auxiliary electrode elements being in alignment with said output electrode in a direction substantially normal to said first alignment, a load circuit connected to said auxiliary output electrode, means for applying signal currents upon said control electrodes, means for varying the potential of at least one of the control electrodes to deflect the electron beam from said output electrode to said auxiliary electrode, said varying means consisting of a rectifier of signal currents, and connections to apply rectified voltage to said one'control electrode.

WALTER. VAN B. ROBERTS. 

